This invention relates to apparatus for displaying various process quantities of a plant with a pictorial display device for monitoring the state of the plant.
Various types of apparatus have been used for monitoring the state of a plant, for example an electric power generating plant, and in some cases the state of the plant is monitored by pictorially displaying the process quantities. The pictorial display of analogue quantities is made with digital values or made in the form of a graph with analogue values. The digital values display is advantageous since the plant operator can recognize the displayed values at a glance. However, as the displayed values vary in response to a small variation of the values the operator pays more attention than needed to such variations. Furthermore, each time the value to be displayed varies, it is necessary to renew the pictorial display thereby increasing the burden on the display apparatus.
To prevent this difficulty, where it is necessary to renew the display of a process quantity, it has been the practice to compare a value now being displayed with a detected value of the process quantity so as to prevent renewal of the display so long as the difference is smaller than a definite value. Thus, the difference between the displayed value and the detected value is compared with a predetermined definite value and the display is not renewed as long as the resulting difference is less than a predetermined value. However, as the absolute value of the process quantity increases, even though the rate of change of the process quantity may be small, the difference between the displayed value and the detected value would become large and in some cases exceed the predetermined value. For this reason, even when the rate of change of the detected value with reference to the displayed value is small, when the absolute value of the process quantity increases the renewal of the display takes place frequently with the result that the burden of the process quantity display apparatus would not be decreased. For example, if the process quantity fluctuates between 399,995 and 400,005 for some reason and when said predetermined value is 10, due to a small variation the detected value would change around 400,000 so that all of the 6 digits change. In other words, the operator pays more attention than is actually needed thus failing to accurately monitor the plant.
Since the variation or fluctuation of the process quantity is inevitable, this problem should be solved when displaying digital values.